1. Field of the Invention
The present invention relates to the transmission of information over a digital subscriber line (DSL).
2. Description of the Related Art
The use of digital subscriber line (DSL) technology has increased in recent years, allowing high-speed (i.e., broadband) access from a central location, typically a telephone central office, to multiple subscribers in homes or offices. The particular advantage of DSL over various other forms of broadband communications is that DSL allows the use of existing copper telephone lines from the central office to the end user, often referred to as the xe2x80x9clocal loopxe2x80x9d, or simply xe2x80x9cloopxe2x80x9d. These existing lines are relatively small gauge (e.g., 24 or 26 gauge) twisted pair copper conductors, and were originally intended for use with a single analog voice channel occupying a relatively low bandwidth of about 3 to 4 kHz. However, the use of quadrature amplitude modulation (QAM) has allowed higher-speed digital communications over these lines, typically allowing data rates on the order of several hundred kilobits per second (Kbps). In addition, most recent DSL work has been concentrated in the area of discrete multi-tone (DMT) technology, which allow multiple channels to be sent over modulated carriers that are spaced apart in frequency; see, for example, U.S. Pat. No. 3,511,936. That technique can increase the data rate to over 1 megabits per second (Mbps). A typical DSL application is for connecting the subscriber to the Internet via the telephone central office, but other uses are possible.
There are numerous implementations of the above techniques that allow for sending data at a variety of rates over various distances, with shorter line distances generally allowing for higher data rates. For example, ADSL (Asymmetric Digital Subscriber Line) allows for a higher transmission rate from the central office to the subscriber than vice-versa. ADSL also typically provides for retaining the lower frequency portion of the phone line for analog voice communication, often referred to as POTS (Plain Old Telephone Service), while the higher frequencies are reserved for data. The data rates are typically about 1.5 Mbps downstream, (i.e. from the network to the end user""s PC) and 384 Kbps upstream (i.e. from the end-user""s PC to the network) in the standard xe2x80x9cG.Litexe2x80x9d version of ADSL.
Referring to FIG. 1, a typical ADSL network topology is illustrated. A central office 100 includes a POTS splitter 101 that interfaces with the twisted pair subscriber line 108, and provides an analog signal to the POTS line care 102 and a digital signal to the ADSL line card 103. The POTS line card in turn interfaces with the voice switch 104, for example a 5ESS(copyright) electronic switch from Lucent Technologies Inc., which provides switched circuit access to the telephone network 106. The ADSL line card interfaces with the IP (Internet Protocol) router 105, which provides switched packet access to the Internet 107. The telephone subscriber line 108 connects to a subscriber 113, typically a home or office, and terminates in a POTS splitter 109, which provides voice frequencies to the telephones 111 and 112, and provides the data over higher frequencies to a personal computer (PC) 110. Note however that the G.Lite standard allows for eliminating the POTS splitter, by spacing the data frequency that carry the digital data sufficiently far in frequency from the lower analog frequencies that interference to the analog voice service is minimized. Then, the PC 110 and phones 111 and 112 may be directly connected to the subscriber line 108, although the use of low-pass filters for the phones 111 and 112 is recommended in some installations.
Referring to FIG. 2, an illustrative G.Lite subscriber line frequency spectrum is illustrated, wherein the analog voice frequencies 200 lie below the downstream (slow) data channel 201 and the upstream (fast) data channel 202. Both of the data channels comprise individually modulated tones that produce 256 bands approximately 4.3125 kHz wide (e.g., 203 and 204). The information in each band is allocated during modem training, so that more information is sent in a given band when the signal-to-noise ratio (S/N) is high, and less information is sent when the S/N is low. In this manner, the rate of information transmission may be maximized for a given subscriber line in the presence of phase and frequency response variations, cross talk, reflections, and noise from a variety of sources.
It is also known in the art to make use of the xe2x80x9canalogxe2x80x9d portion of the voice spectrum 200 for low-speed modems of the V. series (e.g., V. 32, V. 34 and V.90). The information transmitted or received by a modem may originate in digital form, for example computer files or Internet data, or in analog form, for example speech, that is converted to digital form. The modulation type depends on the particular V. series modem in use, but these modems are collectively referred to as xe2x80x9canalogxe2x80x9d modems because they utilize the POTS frequency portion of the telephone line; that is, below approximately 4 kHz. This limits the data rate to about 56 Kbps or less, which is referred to as xe2x80x9clow speedxe2x80x9d herein.
In order to establish high-speed DSL communications over the phone line, the modems at both ends must engage in a start-up sequence, often referred to as xe2x80x9chandshakingxe2x80x9d. The various handshaking protocols depend on the variety of DSL in use, but usually determine at least the line loss, frequency and phase response of the line, so that proper line equalization can be achieved. The information determined during handshaking can also be used for echo cancellation purposes and reduction of certain types of cross talk. However, over certain loop conditions, the DSL modems may encounter difficult loop impairments. In some cases, the modems on both sides of the loop may not be able to establish a connection. To solve this problem, two engineers, one on either side of the local loop cable, communicate with each other via a separate phone line in order to pass diagnostic data. However, that technique is slow and does not allow the xe2x80x9cautomaticxe2x80x9d handshaking mode to work. Furthermore, the intervention of trained personnel increases the time and expense required to establish the DSL service.
I have invented a technique and apparatus for passing startup information between DSL modems connected by telephone lines. In my invention, DSL modem startup parameters are passed in digital form over the voice band channel. For this purpose, analog V. series modems may be used to communicate over the voice band channel.